Management of the volume of tailings, aqueous solutions, emulsions or suspensions of nonvolatile salts (“brines”), organics or solids are relevant to many industrial sectors. Applications with salts such as sodium chloride, calcium chloride, sodium carbonate, potassium carbonate (potash), mixed salts (saltcake), etc. include, but are not limited to, underground hydrocarbon storage in salt cavems, production of table or industrial salt, solution salt mining or disposal of run-off, process or reject streams from desalination plants, mining, petroleum production/refining, or other processes. Applications with aqueous organics such as non-volatile alcohols, glycols, amines, acids/sulfonates, organic salts include, but are not limited to, ice/hydrate control, drying/humidity control, heating, ventilation and air conditioning (HVAC), Biox solids or organic salt production. Applications with tailings, comprised of sand, clays, salts, metals, radioactive materials, hydrocarbons include, but are not limited to, mineral mining, bitumen/heavy oil production, or heap leaching. In each case, there is an incentive to remove all or a portion of the water from the aqueous phase in the most economical manner possible.
In the case of tailings ponds generated from oil sand activities, the goal is to dewater the tailings to turn the ponds into a trafficable landscape that is solid enough to allow for motorized traffic and eventually, land reclamation. Currently in Northern Alberta tailing ponds cover about 180 km2. While some of the water is released and recycled from the ponds to be reused in oil sand processing, the majority remains as mud almost indefinitely. Due to the nature of tailing ponds, there are serious challenges faced by the oil sand industry, from both environmental and economic perspectives. Currently, there are no commercial tailings management technologies that are allowing for efficient and timely reclamation of the tailings ponds back to boreal forest or equivalent land capability.
The rain dilution of brine ponds in cool humid climates such as Sarnia, Ontario has been a problem seeking a solution for more than 20 years. The balance between passive evaporation and rainfall can be approximated by the “pan evaporation rate” and precipitation as measured and reported in US, Canadian and other government meteorological data. There are many government, industry and academic charts and graphs available that clearly indicate that cool humid climates are not suitable for brine pond evaporation
Different types of ponds present various challenges in the management of their volumes.
Tailings Ponds
The tailings generated from the bitumen extraction process are typically stored in above-ground settling basins (‘ponds’). When tailings are released to a pond, large solid particles (mainly sand) settle to the bottom while water rises to the top. During this process, a middle ‘mature fine tailings’ (MFT) or ‘middlings’ layer, composed of water-suspended fine clay particles, is created. The challenge is that MFT/middlings take years to settle, which hinders water extraction from the ponds and subsequent drying of tailings and consequently, results in extensive delays to land reclamation. Following several decades of MFT settling, the dried land can eventually, e.g. in 30 years, be covered by sand and topsoil to enable a reclaimable area containing both upland and wetland features.
Oil sands tailings ponds are constructed with containment dykes and groundwater monitoring facilities in order to capture and recycle run off water and minimize seepage. Currently, a portion of the ‘free’ water from the tailings ponds is recovered and reused in the bitumen extraction process, while the remainder is left in the ponds to allow the mature fine tailings to settle over time. In order to protect the quality of river water, it may be necessary to ensure that no water that has been used in the bitumen extraction process is returned to rivers. Instead, the used water may need to be transferred to tailings ponds and then recycled into the production process.
Brine Ponds
Sub-saturated brine will dissolve salt from the walls of storage caverns and hence alter the shape and integrity of the storage facility in an uncontrolled manner. Several caverns in the Sarnia area of Ontario, Canada, for example, have been adversely affected by “solution salt mining” due to sub-saturated brine over decades of use. Sub-saturated brine occupies additional surface storage volume, and when surface storage is full, it must be stored in the cavern, reducing the cavern volume available for hydrocarbon storage.
The cost of brine disposal continues to increase, and is currently at about US $3/bbl with “take or pay” minimums. In cool/humid climates, the volume requiring disposal may be increased by a typical factor of four or five as a result of rain/snowfall dilution. In addition, current brine disposal outlets are subject to third party acceptance terms and processes and potential regulatory constraints, and are not considered to be secure long term.
In some instances, it is feasible to use overhead roofs or floating covers to isolate the brine storage facility from atmospheric precipitation and absorption at high humidity. The practicality of using covers or roofs is limited by the size (surface area cost), shape (regular versus irregular), location and number of ponds. Pond surface covers still require a system to collect water from the top of the cover, and dispose of the water in accordance with increasingly stringent environmental regulations. The recovered water may still be contaminated enough to require subsequent treatment prior to disposal and may still require disposal as brine even if only slightly contaminated with brine. Floating pond covers carry a high risk of damage during use or transport into adjacent areas during wind storms, hurricanes or tornadoes. As a result, pond covers are not commonly used, and it is generally more economical to dispose of rainwater diluted brine into industrial or potable salt production. This generally involves a fee for processing in addition to the operating and transportation costs, all borne by the producer/seller of the excess brine. These outlets are not considered to be secure, and are subject to re-negotiation every few years.
In some instances, excess brine volumes are managed by injection into porous underground aquifers that are deep and salty. This method depends on aquifer availability, and the proper approvals being in place to permit injection. In other cases, underground disposal wells are employed, but this also requires an available well, and a permit to inject. The high cost of drilling coupled with high probabilities of accessing a formation with poor disposal characteristics may render this option unavailable in practical economic terms and the substantial distance to the next closest suitable disposal formation could require a pipeline or an increase in trucking cost, making this unattractive.
Brine flash evaporators have been employed but are expensive to operate and are susceptible to fouling and corrosion. Membrane technologies are technically feasible, but suffer from high capital and operating costs (high pressures for reverse osmosis, and the cost of equipment installation for direct osmosis). Other methods to manage local brine volumes include transfer to another surface location and shipment for ocean disposal are not normally feasible for inland areas but resort may be made to this option if containment becomes sufficiently critical to justify the high cost.
Various studies and proposals on evaporation have been made. El-Dessouky, H. T. et al, “Evaporation Rates from Fresh and Saline Water in Moving Air”, Industrial and Engineering Chemistry Research, 41, 642 (2002), for instance, reports the results of studies on the named topic. Smith et al. have reported two series of studies in C. C. Smith et al., “Measurement and Analysis of Evaporation From an Inactive Outdoor Swimming Pool”, Solar Energy, 53, 3 (1994 and C. C. Smith et al., “Rates of Evaporation from Swimming Pools in Active Use”, ASHME Transactions: Research, 104(1), 514 (1998).
US 2013/0175223 (Rennard et al) discloses a method of remediating slurry ponds by distributing geotextile or geotubes over the surface of the pond and placing a sand load on these materials to allow supernatant water to flow to the top and separate out of the sludge layer.
US 2013/081298 (Bugg et al.) discloses a method to improve the dewatering and drying of mature fine tailings in oil sands by the addition of flocculants: the flocculated fine tailings are deposited on a deposition cell with a sloped bottom surface to allow drainage of released water.
US 2013/112561 (Jajuee et al.) describe the design and use of electrokinetic thickeners having a voltage gradient to dewater slurries, including tailings.
CA 2776389 (Betzer et al.) describes a method of utilizing heat to generate steam (i.e. separating water) from a mixture of water, solids and organics. The described method entails using a hot driving fluid to heat water containing solids and organics to separate solids and produce steam that can be used for underground injection or for generating hot process water but does not address the dewatering/drying of tailings.